Data signals carried by lossy media such as twisted-pair copper wire are subject to amplitude and phase distortions which are frequency and cable length dependent. Uncompensated, this results in both amplitude and timing jitter, such as intersymbol interference, (ISI) which imposes practical limitations on the attainable bit error rate (BER) performance after reception. Amplitude and phase equalization is typically employed to correct the distortions.
Adaptive receiver equalization techniques have been employed in the highest speed data communications networks such as ATM (155 MBits/s), 100BaseTX Ethernet (100 MBits/s) and FDDI TP-PMD (Twisted-Pair FDDI, 100 Mbits/s) to allow for transmission on up to 100 m of Category 5 Unshielded Twisted-Pair (UTP5) and Shielded Twisted-Pair (STP) at BERs exceeding 10E.sup.-10 with radiated emissions within FCC Class A and Class B limited.
Many, if not all, available solutions for receiver adaptive equalization use the received voltage amplitude and a fixed amplitude reference to control equalization. Given a tight tolerance on the transmitted amplitude and fixed amplitude reference proper equalization can be attained. However performance is degraded as tolerances increase.
Because of the required transformers for system isolation and radio frequency emission control, baseline wander degrades performance as well, if transmitted data is not strictly balanced. If positive or negative peak detection schemes are used to extract amplitude information, baseline wander will cause an apparatus shift in received amplitude thereby introducing an error in the required equalization. If equalization is fixed, the baseline wander will not impact equalization but will reduce the decision threshold noise margins at the data quantizer, impacting BER performance.
In data communication systems which are subject to large baseline wander components, baseline wander correction is required to maintain the required BER. However, even small residue errors in this correction circuit can introduce an amplitude error into the adaptive equalizer path which can degrade performance.